1) Field of the Invention
The present invention relates to a display apparatus for motor vehicles in which a displayed image on a display device is reflected by a reflecting member and led to a viewing point of a driver so that the displayed image of an indicator can be recognized by the driver as a virtual image behind the reflecting member.
2) Description of the Prior Art
There has been available a display apparatus for motor vehicles wherein an indicator image can be observed as a virtual image at a remote location in front of the vehicle, in which a display device made up of fluorescent display tubes, liquid crystal display elements for displaying information, or the like, is installed in a dashboard by facing a display side upwards. The image light emitted from the display device is passed through an opening of the dashboard and reflected by a reflecting member, such as a windshield, toward the viewing point of the driver, so that the virtual image of the displayed image is visible by the driver behind the reflection member, i.e., in front of the vehicle.
In this type of display apparatus, however, since the image light emitted from the display device is reflected at two points, i.e. at inner and outer surfaces of the windshield, a ghost image is produced.
To eliminate such problems, it has been proposed to employ such configurations as disclosed in Japanese Patent Application Laid-Open Publication Nos. Heisei 2-294615 and 2-141720, wherein the display device is arranged to polarize the image displaying light in S-polarization, and a polarization direction adjusting means, which changes the incoming S-polarization light into a P-polarization light, is interposed between laminated glass plates which forms the windshield, so that no unwanted reflection may take place at the outer surface of the windshield and the ghost image has been prevented from occurring.
However, in this type of display apparatus, since a screen of the display device is faced upwards, external light rays such as rays of sun light incident upon the screen of the display device through the opening of the dashboard have caused a so-called washout phenomenon which makes the display very difficult to see.
In order to eliminate the problems encountered with the prior art display devices, the inventor has proposed a configuration shown in FIG. 2. In this configuration, immediately above the display surface of the indicator or display device 1, provided are first and second prisms 11, 12 having the same apex angles .theta. and stack them together so that their apex angles .theta. of the prisms are directed opposite directions to each other.
According to the configuration above, when there is an external light from the windshield 4 incident to the indicator 1 through the opening 2a of the dashboard 2, it is reflected at an interface 11a of the first and second prisms 11, 12 stacked together, and directed to a direction shown by an arrow F, so that there will be no external light incident to the display surface of the indicator 1. Hence, there is no possibility for the display image of the indicator to be washed out by the external light.
Further, since the prisms 11, 12 of the same shape are stacked together in opposite directions to each other, the image light emitted from the indicator 1 is hardly affected when it passes through the prisms. This ensures the viewer that the displayed image visible from the viewing point 6 is not distorted, and allows to recognize a virtual image behind the reflecting member 3 or in front of the motor vehicle.
No problem may be encountered with the configuration proposed by the inventor, however, in the apparatuses disclosed in the Japanese Patent Application Laid-Open Nos. Heisei 2-294615 and 2-141720 wherein the windshield is used as a reflecting member, it may cause a problem. In the latter said apparatuses, if the aforesaid prisms are employed in order to prevent unwanted phenomenon from causing by the external light, the S-polarization light emitted from the display device 1, which device consists of a liquid crystal display element 1a with a backlight 1b, is reflected with higher reflectance as compared with that of normal light at the prism 12 and air interfaces A, B and that at the prism 11 and air interfaces C, D. This results in the decrease of the intensity of the incident light to the windshield 4. Hence, the brightness of the displayed image is decreased.
The phenomenon described above can easily be understood from graphs of FIGS. 4 and 5. That is, the above phenomenon is caused by the fact that the incident light from the air having the refractive index 1 into the prism having the refractive index 1.5 has the same reflectance for S-polarization light and P-polarization light up to the incident angle of approximately 30.degree., as shown in FIG. 4; however, the reflectance becomes higher for the S-polarization light than the P-polarization light when the incident angle is at or above 30.degree..
On the contrary, as shown in FIG. 5, when the light enters from the prism having the refractive index of 1.5 into the air having the refractive index of 1, the reflectance for the S-polarization light and P-polarization light are equal up to the incident angle of approximately 15.degree.. However, for greater incident angles up to the total reflection angle of about 42.degree., the S-polarization light has a higher reflectance than that of the P-polarization light. Therefore, when the prisms are arranged in such configuration as shown in FIG. 3, a transmittance of the incident light to the prism for the S-polarization light will be decreased to 30-40%. In case of the P-polarization light, the reduction of the transmittance will be of about 10%.
In FIGS. 4 and 5, ob (=56.3.degree., 33.7.degree.) is the Brewster angle which is the angle of incidence at which no reflection of the P-polarization light will occur. Further oc (=41.8.degree.) indicates a total reflection angle.